In common rotary drilling methods and systems used in drilling oil field boreholes, power rotating means delivers torque to a drill pipe, a plurality of which forms a drill string, via a kelly and a rotary table. The drill string in turn rotates a bit located at its lowermost end that drills a borehole through the sub-surface formation. The drill string is supported for up and down movement by a drilling mast located at the earth's surface. A drill line or cable supported by the drilling mast and coupled to the drill string is used in conjunction with a rotating drum to facilitate the up and down movement. The drill line is anchored at one end called the dead line anchor, which is typically located adjacent to one leg of the drilling mast. The drill line extends from the anchor upwardly to a crown block formed of a plurality of rotatable sheaves at the top of the mast. The drill line is reaved around the sheaves in the crown block and extends downwardly between the crown block sheaves and rotating sheaves in a traveling block. The drill line then extends from the crown block downward to a rotating drum or drawworks. that moves the crown block up and down by reeling the drill line in or out.
As will be appreciated by those skilled in the art, determining and monitoring the depth at which a component of the bottomhole assembly (BHA) is located at any given time in a wellbore is important for many reasons. For example, the drilling rig operator needs to know the depth at which the bottom hole assembly is located during trips in and out of the well so that he can be cautious when passing through sensitive zones such as bridges, ledges, or key seats. In addition, by correlating information gathered from offset wells, a driller needs accurate depth measurement information while drilling subsurface formations to anticipate trouble zones, e.g., high gas-pressured gas zones, in order to take appropriate precautionary measures. Also, accurate depth information is extremely valuable when performing directional or horizontal drilling operations.
In recent years, many developments have been made in the area of gathering borehole data while the drilling operation is being conducted. These services, which are commonly referred to as measurement-while-drilling (MWD), logging-while-drilling (LWD), and formation evaluation while drilling (FEWD), typically incorporate various sensing devices into the bottomhole assembly to gather information related to, for example, formation lithography, downhole environment, and tool operating parameters. The raw or processed data gathered by such devices are typically either transmitted to the surface in "real time" by using, for example, a mud pulse telemetry system, or stored in a memory device located in the downhole tool for later retrieval when the BHA is brought back to the earth's surface, or simultaneously transmitted in real time and stored downhole. For much of this information to be of significant value, particularly lithography data, it must be correlated to the particular depth at which the information was obtained. Accordingly, it is extremely important for MWD or LWD service providers to have an accurate depth measurement system and apparatus.
Present day depth systems and methods typically include a combination of keeping a tally indicating the length of each drill pipe inserted into the borehole, and measuring the incremental length of the last drill pipe being lowered into the borehole during the drilling or tripping operation by monitoring the movement of the traveling block. Traveling block movement is commonly determined by monitoring the motion of the drilling line as it is fed from the drawworks, e.g., with a sensor coupled to the rotating drum or one of the sheaves in the crown block. This general type of system, however, contains many sources of errors and inaccuracies. For example, the length of a particular pipe section is simply inaccurately measured or noted erroneously, or added to the drill string in an order different from that noted in the tally. In addition, with respect to monitoring the motion of the drilling line through drum rotation to record the length of the last pipe, since the drill line cable stretches over time and because the cable is wound in layers around the rotating drum, the rotation of the drum itself does not accurately correlate to the length of the last drill pipe being lowered.
Further inaccuracies with prior methods typically occur during the procedure when pipe is added or subtracted to the drill string either while conducting the drilling operation or while tripping in or out of the well. For example, when the rig's traveling block has reached its maximum downward movement during a drilling operation and a new section of pipe must be added, the traveling block and connected drill string are first raised a short distance by reeling in the drill line cable, followed by placing slips in the rotary table. After the slips are inserted, the traveling block is lowered a short distance such that the slips support the drill string, which allows the kelly to be unscrewed. In the process, cable is reeled out while the BHA remains stationary. The disparity in movement is due to the release of tension in the cable since the cable is no longer supporting the weight of the drill string. On the other end of the procedure when the kelly is swung over to the pipe and the new pipe is attached onto the kelly, and the kelly and new pipe are swung back and attached to the drill string, the traveling block first moves upward to a point where the slips can be removed. When the slips are removed, again misallocations regarding drum rotation and traveling block movement with respect to the drill string movement are made with resulting depth determination inaccuracies. These small errors at each transition can translate into an accumulated error of several feet during the course of drilling a well.
An additional problem with tracking BHA position based on traveling block altitude is that such systems, for a variety of reasons, often loose track of the block position. Systems that determine block position based on encoders connected to the drawworks loose block position accuracy, for example, because of cable stretch over time and changes in the way the cable wraps on the drumwork's rotating drum. Systems that place encoders on the fast sheave in the crown block typically loose block position accuracy, for example, because of cable slippage and cable stretch. Both of these general types of systems typically lack a reliable way of resetting block position that does not affect or interfere with the drilling operation.
In order to overcome some of the inaccuracies inherent in most prior art depth techniques, several different methods and apparatus have been proposed. For example, in U.S. Pat. No. 4,114,435 to Patton et al, it is proposed to measure different traveling block reference points that relate to when the cable on the drawwork drum reaches different layers of unwinding, and then to determine the location of the traveling block via an equation, the reference points, the rotation of the drum, etc. The Patton et al system, however, still provides inaccuracies because it fails to account for the dynamic nature of the cable layering process. Moreover, an account for the cable stretching over time is not provided for.
U.S. Pat. No. 4,787,244 to Mikolajczyk proposes to automatically determine the drill bit depth by tracking the movement of the cable. Movements of the cable are only tracked when the weight carried by the traveling block exceeds a certain minimum threshold as determined by a tensiometer on a cable. However, this prior technique fails to account properly for movements of the cable during the slips-in and slips-out procedure when the transition is made through the threshold. Similar types of errors are believed to be inherent in the system proposed in U.S. Pat. No. 4,616,321 to Chan.
U.S. Pat. No. 4,610,005 to Utasi proposes a video system that monitors the position and movement of the traveling block to determine borehole depth. In Utasi's system, a video camera is positioned to track the vertical movement of the traveling block. However, Utasi's systems seems to be fairly impractical and inaccurate because of the remote distance that the camera must be positioned to view the entire rig. In addition, the distance between the camera and the rig renders the system susceptible to interference from the rig structure, lighting changes, equipment movement, etc.
In light of the above, a principal object of the present invention is to provide a system for and method of accurately determining and monitoring the depth at which a bottomhole assembly is located within a wellbore.
A further object of the present invention to provide a system for and method of accurately measuring and recording the length of an object before it is inserted into a well.
Another object of the present invention is to provide a means for verifying and resetting a depth determination to substantially reduce accumulated errors.
A further object of the present invention is to provide a system for and method of accurately measuring and recording the depth at which a bottomhole assembly is located while substantially not affecting or interfering with the normal operation of a drilling rig and its crew.